The invention concerns a method for quality assurance when processing materials with laser energy, and preferably laser beam welding or cutting. In such processing, the ultraviolet light arising in the plasma cloud during the material processing operation is detected by a detector in a wavelength of approx. 200 nm up to approx. 450 nm to control the laser beam coupling in the workpiece as well as the check for specified limits of other process parameters such as laser output power, beam defocussing, beam quality, feeding of shielding gas and working gas as well as workpiece structure, cleaning status of the workpiece surface, and welding gap width. Such a quality assurance method is known from DVS-report No. 113 on the conference "ECLAT '88", p. 58-59. Known devices use a light sensitive detector consisting of a silicon photodiode, which has an enhanced UV-light sensitivity, and an optical filter, which is transparent only for the particular UV-light wavelengths. The filter is located between the welding pool (4) and the detector. The received UV-signal is turned into an electric signal by the detector, and is fed to a processing unit. With such a device, it is possible during a laser material processing, to detect changes of the various parameters and identify them as possible material processing faults.
This known device has the disadvantage, in that it is not possible to detect and indicate a processing fault during a laser welding process, when two or more parameters with contrary effects on the welding plasma have changed at the same time. Furthermore, if a high detection sensitivity is adjusted, such a device indicates too many faults, being in fact the result of a laser welding process parameter disturbance but not the result of welding failures.
As background of this invention also the use of a photodiode for monitoring a laser drilling process was considered. This well known monitoring method is shown in: JP-DS "Pat. abstr. of Japan", 1987, Vol. 11/No. 166, M-593, Kokai No.: 61-296980.